High-voltage metal-clad apparatus comprising a capacitive voltage divider

ABSTRACT

High-voltage metal-clad apparatus comprising a pressurized grounded metal enclosure and an axial high voltage conductor. A capacitive voltage divider cooperates with a section of the conductor that can be isolated to deliver a low voltage output signal. A movable probe can apply a test voltage to the conductor to calibrate the voltage divider in situ. The probe is voltagesupplied through a bushing traversing the enclosure and rated only for a fraction of the rated high voltage of the apparatus.

United States Patent 174/11 Bl-l; 339/177 R, 147 R, 147 P Amalric 1'Oct. 31, 1972 [54] HIGH-VOLTAGE METAL-GLAD [56 References CitedAPPARATUS COMPRISING A CAPACITIVE VOLTAGE DIVIDER UNITED STATES PATENTSt z m I 2,199,757 5/1940 Rohde ..324/126 .[72] l E c 2,805,399 9/1957Leeper ..339/171 [73] Assignee: Merlin Gerln, Grenoble, France v PrimaryExaminer-Rudolph V. Rolinec [22] 1971 Assistant Examiner-Ernest F.Karlsen [21] Appl. No.: 168,694 Att0rney- -Stevens, Davis, Miller &Mosher v 57 ABSTRACT {30] Foreign P 'k Data, High-voltage metal-cladapparatus comprising a pres- Aug. 6, 1970 France...'. ..7029159 surizedgrounded metal enclosure and martial high 7 I D voltage conductor. Acapacitive voltage divider [52] U.S. Cl. ..324/126, 174/11 BH, 324/74,cooperates with a section of the conductor that can be v I 339/177 Risolated to deliver a low voltage output signal. A [51] Int. Cl. ..G0lr1/20, G01r 35/02 movable Probe can pp y a test voltage to the conduc-[58] Fi ld of S h 324/12 74, 149, 725 153 p; tor to calibrate thevoltage divider in situ. The probe is voltage-supplied through a bushingtraversing the enclosure and rated only for a fraction of the rated highvoltage of the apparatus.

5 Claims, 4 Drawing Figures HIGH-VOLTAGE METAL-CLAD APPARATUS COMPRISINGA CAPACITIVE VOLTAGE DIVIDER in urban centers.

Conventional measurement transformers or capacitive dividers have beenincorporated in these stations in order to measure the voltage of thescreened network with the necessary precision and to be able to controlall protective relays. In the case of capacitive dividers, recourse hasbeen had to a plurality of capacitors of relatively high capacitancewhich have been connected in series between the high voltage conductorand the jacket in order to tap off a reduced voltage from a part of thecapacitors. These conventional magnetic transformers and capacitivedividers are bulky and expensive so that the manufacturers havecontemplated their replacement by a device which is more adapted to thenature of screened stations. It was found that the concentricarrangement traditionally adopted for the different members of thestation is also possible for a capacitive divider It is sufficient toarrange a cylindrical electrode between the axial conductor and theinner wall of the jacket and to cause to extend from the jacket, via aninsulating lead-through, a conductor which is connected to saidelectrode on the one end and to one of the terminals of an outercapacitor, the

other terminal of which is connected to the jacket, on the other end, inorder to have a faithful image of the high voltage of the axialconductor.

As a matter of fact, if the said electrode which constitutes a plate ofthe capacitive divider is suitably arranged between the axial conductorand the jacket, the

low voltage which appears at the terminals of the outer capacitorfaithfully reproduces the high voltage to be measured by beingproportional to it.

' The increase in size of the screened station due to the presence ofsuch a capacitive divider is only slight, if any, and the cost of thisdivider is far less than that of the traditional types. Unfortunately,the capacitance of the inner electrode with respect to the axialconductor is very low of the order of a few picofarads so that the lowpower level resulting therefrom, particularly with respect to theconsumption of the protective relays, compels the user to have recourseto electronic amplifiers of high current gain capable of delivering anoutput voltage proportional'or identical to the input voltage tappedfrom the outer capacitor, but under low impedance.

A high precision of measurement can easily be obtained with thisarrangement provided that one calibrates the ratio of the capacitivedivider with amplifier connected, which may be effected by applicationof a known alternating voltage to the axial conductor, previouslyinsulated from the network, and by thereupon measuring the voltage atthe input and output of the amplifier. However, while for a givenarrangement this calibration can possibly be effected before the fillingof the envelopes with the compressed gas, its value, in view of the highdegree of precision required for the measurement, becomes doubtful afterthe final mounting of the envelopes and the filling of them with thecompressed gas from which there can easily result slight displacementsor deformations of the constituent elements of thecapacitive dividerresulting in a modification of the capacitances. The same is true of theouter components and in particular of the amplifier, the aging andpossible replacement of which make the initial calibration veryunreliable.

The adoption of the said divider therefore depends on the possibility ofcalibration during the use of the installation. Now, the inaccessibilityof the parts under voltage of a screened station of the type mentionedhas not made it possible up to now to process with-such calibrations inoperation and for this reason the measurement system with capacitivedivider with concentric electrode has never found application, despiteits simplicity.

One could contemplate calibration of the system by means of a voltagetransformer connected to the supply cable of the station in question.Unfortunately, the core of this cable is frequently accessible only at agreat distance from the screened station, for instance severalkilometers away in the case of urban stations fed by an external stationwith overhead lines located outside centers of population. One then hasno certainty that the voltage measured at a remote distance correspondsto the voltage at the place of the screened station.

' The object of the present invention is to create a screened stationcomprising a capacitive divider of the type mentioned having simple,strong and inexpensive means for permitting the rapid andprecisecalibration of the divider without affecting the hermetic natureof the metal envelopes and without therefore making the emptying thereofnecessary.

In the screened station in accordance with the invention the saidenvelope comprises an insulating leadthrough insulated for a fraction ofthe nominal voltage of the station in order to permit a moving contactto reach the said conductor without destroying the seal of the envelopeat a point of the conductor capable of being insulated from the networkin order to apply to it a divider calibrating voltage.

In accordance with one development of the invention, the said movingcontact consists of the moving contact of a grounding switch which isduly insulated from the said metal envelope.

The fact of insulating the said contact from the envelope for only afraction of the world rated voltage of the station, for instance for 10kV in a 220 kV installation, makes it possible to obtain the desiredresult with inexpensive means and small size.

In FIG. 1, a high-voltage conductor is arranged in the axis of acylindrical metal envelope or jacket 12.

The specific formation and arrangement of this envelope and of thisaxial conductor do not constitute part of the invention and willtherefore not be described in detail. It is sufficient to point out thatthe jacket may be formed of compartmented'or non-compartmented sections.The enclosure defined by the jacket 12 is filled with a compressed gashaving a high dielectric strength and preferably also a higher areextinguishing power in the event that the station comprises screeneddisconnecting switches such as section switches or circuit breakers.This gas may be an electro-negative gas such as sulfur hexafluoride (SFor simply compressed air.

Two screened disconnecting switches 14 and 16 make it possible toisolate the central section of the conductor 10 which is supportedwithin the jacket 12 by suitable insulators (not shown) which do notform part of the invention. These screened disconnecting switches may beswitches with axially movable contact rod, but switches of a differenttype can be provided. Their envelopes may be arranged in the extensionof the envelope 12 of the central section'or else at an angle (forinstance 90) with respect to the latter. For the isolating of thesection in question of the conductor 10 there will preferably be usedswitches normally specified for the installation, located at asufficient distance from said section notto introduce any appreciableerror in the calibration. The jacket 12 is grounded in the customarymanner.

An annular metal screen 18 is arranged concentrically between theconductor 10 and the inner wall of the jacket 12 in the vicinity of saidwall but electrically insulated from it. An insulating lead-through 20arranged in the jacket 12 supports on the one hand the screen 18 and onthe other hand a conductor 22 arranged in its axis to make it possibleto tap off the voltage of the electrode 18 and apply said voltage to oneof the terminals of an external measuring capacitor 24 whose otherterminal is grounded at 26. The conductor 22 is connected to the inputof a suitable amplifier 28 which in known manner feeds measuringinstruments, protective relays, etc. In accordance with the invention, asecond insulating lead through 30 passes through the jacket 12 to permita moving contact 32, by a radial movement with respect to the envelope,to

-reach the conductor 10 in order to apply to it a predeterminedcalibration voltage without interrupting the seal of the jacket 12. Thiscalibration voltage is substantially less than the nominal operatingvoltage of the station, which makes it possible to provide a leadthroughinsulator 30 of reasonable size and cost.

The amplifier 28 is necessary in order to be able to provide an outputvoltage which is identical or proportional to its input voltage,supplied by the capacitor 24, but at low impedance. As a matter of fact,since the capacitance of the screen 18 with respect to the conductor 10is very low, the impedance of the divider comprising said capacitanceand that of the capacitor 24 (with the plate 18 in common) is very highso that the divider is incapable of feeding receivers of relatively lowimpedance.

This device operates in the following manner The moving contact 32 beingnormally in its withdrawn position, it is brought onto the conductor 10after the central section of the high voltage axial conductor 10 hasbeen electrically isolated from the network, by the opening of the twodisconnecting switches 14 and 16, which are suitably selected.

A suitable alternating voltage (for example of the order of 5 percent ofthe rated voltage) is then applied to the closed contact 32 andtherefore to the conductor 10, and by means of a measurement bridgeprovided with a zero indicator and a potentiometer, the ratio of .thevoltages at the terminals of the capacitor consisting of the plate 18and the conductor 10 and at the terminals of the outer capacitor 24 isdetermined in knownmanner. The calibration having been effected, thecontact 32 is removed and the installation is ready or again ready tooperate normally.

It is advisable to use an amplifier 28 which has a gain control systemwhich makes it possible to adjust the input voltage/output ratio to apredetermined rated value during the calibration.

The ratio measured will characterize the entire capacitive divider aslong as nochange is made on the capacitances in question, including theinput capacitance of the amplifier 28.

In case of the replacement of the amplifier or of any change made in thestation which might result in a modification of the other capacitancesin question, a recalibration will be effected before the station isagain placed in operation.

FIGS. 2 to 4 show various possibilitiesfor the execution of thecalibration switch 32 with lead-through 30. In the different figures 1to 4, identical reference numbers designate identical or similarelements.

In FIG. 2, the moving contact 32 passes through an opening 34 providedin an electorstatic electrostatic 36 connected to the conductor 10 andintended to avoid disturbance of the electrostatic field by a fixedcontact 38 which is rigidly connected with the conductor 10 andcooperates with the moving contact 32. The seal of the lead-through 30is formed by at least one toroidal joint 40 and by a valve 42 rigidlyconnected with the moving contact 32 and which places itself, assistedby the pressure prevailing within the enclosure, on a seat provided onthe inner end of the lead-through 30 in open position of the contact 32.

A second equipotential screen 44 surrounds the contact 32 to impose ahomogenized electrostatic field around the open contact 32.

In FIGS. 3 and 4, the lead-through 30 is arranged in the axis of a boss50 of generally frustoconical shape of the jacket 12 which makes itpossible to avoid the screen 44. In FIG. 4, this boss 50 is opposite theend of a jacket 52 at the intersection of two tubules which areperpendicular to each other.

In accordance with a preferred embodiment of the invention, thecalibration switch 30, 32 serves at the same time as grounding switch.For this purpose, it is sufficient to isolate the contact 32 in themanner described above, that is to say for a fraction of the ratedvoltage, and to provide a removable connection 54 (see FIG. 4) such as abraid which makes it possible to connect the contact spindle 32selectively to ground or to the calibration voltage.

Other embodiments can be course be contemplated without thereby goingbeyond the scope of the invention.

What is claimed is:

1. Electrical apparatus rated for a high voltage comprising: i

a. a linearly extending high voltage conductor;

b.- a metal cylindrical enclosure coaxially surrounding and spaced apartfrom said conductor;

c. a pressurized dielectric gas in said enclosure; I

d. capacitive voltage divider means including an electrode interposedbetween said enclosure and a section of said conductor in capacitiveassociation with the latter, a measuring capacitance coupled to saidelectrode so as to acquire a low voltage when said conductor is highvoltage energized, output means connected to said capacitance;

e. means to isolate said section of said conductor;

and

f. calibrating means to accurately determine the reduction ratio of saidcapacitive voltage divider means by application of a relatively low testvoltage to said section, said calibrating means comprising awithdrawable test'probe movable into and out of engagement with saidsection, and insulating bushing means traversing said enclosure andhaving a through conductor electrically connected to said probe to applysaid test voltage through said probe to said section, said busing meansrated predetermined for a voltage substantially lower than saidpredetermined rated high voltage of said apparatus.

2. Apparatus according to claim 1, said bushing means being rated for avoltage substantially not higher than 5 percent of said rated highvoltage.

3. Apparatus according to claim 1, further comprising means to groundsaid probe.

4. Apparatus according to claim 1, said enclosure comprising adome-shaped outwardly directed projection accommodating said bushingmeans.

5. Electrical metal-clad apparatus comprising a. a linearly extendingconductor rated for a predetermined high voltage;

b. a metal cylindrical enclosure coaxially surrounding and spaced apartfrom said conductor;

c. capacitive voltage divider means delivering a low output voltage ofsaid conductor;

d. means to isolate a section of said conductor in capacitive relationwith said voltage divider;

e. a calibrating switch including an insulating bushing traversing saidenclosure and a movable contact slidably mounted in said bushing andmovable into and out of engagement with said section, said bushing beingrated for a voltage substantially lower than said predetermined highvoltage; and

f. means to connect said movable contact selectively to ground and to atest voltage.

1. Electrical apparatus rated for a predetermined high voltagecomprising : a. a linearly extending high voltage conductor; b. a metalcylindrical enclosure coaxially surrounding and spaced apart from saidconductor; c. a pressurized dielectric gas in said enclosure; d.capacitive voltage divider means including an electrode interposedbetween said enclosure and a section of said conductor in capacitiveassociation with the latter, a measuring capacitance coupled to saidelectrode so as to acquire a low voltage when said conductor is highvoltage energized, output means connected to said capacitance; e. meansto isolate said section of said conductor; and f. calibrating means toaccurately determine the reduction ratio of said capacitive voltagedivider means by application of a relatively low test voltage to saidsection, said calibrating means comprising a withdrawable test probemovable into and out of engagement with said section, and insulatingbushing means traversing said enclosure and having a through conductorelectrically connected to said probe to apply said test voltage throughsaid probe to said section, said bushing means rated for a voltagesubstantially lower than said predetermined rated high voltage of saidapparatus.
 2. Apparatus according to claim 1, said bushing means beingrated for a voltage substantially not higher than 5 percent of saidrated high voltage.
 3. Apparatus according to claim 1, furthercomprising means to ground said probe.
 4. Apparatus according to claim1, said enclosure comprising a dome-shaped outwardly direCted projectionaccommodating said bushing means.
 5. Electrical metal-clad apparatuscomprising : a. a linearly extending conductor rated for a predeterminedhigh voltage; b. a metal cylindrical enclosure coaxially surrounding andspaced apart from said conductor; c. capacitive voltage divider meansdelivering a low output voltage of said conductor; d. means to isolate asection of said conductor in capacitive relation with said voltagedivider; e. a calibrating switch including an insulating bushingtraversing said enclosure and a movable contact slidably mounted in saidbushing and movable into and out of engagement with said section, saidbushing being rated for a voltage substantially lower than saidpredetermined high voltage; and f. means to connect said movable contactselectively to ground and to a test voltage.